Bulk venders are widely used for vending a wide variety of merchandise, from confectionaries to toys. Part of the appeal of bulk venders is that they attract very little overhead in terms of both space and labour. Bulk venders are typically located in high-traffic public areas, and require only periodic servicing to collect deposited coins, refill the product storage bin and, occasionally, to repair or replace parts. As such they are ideal for "self-service" sales of small articles.
For the same reasons, however, bulk venders are frequently subjected to abuse, and particularly to attempts to defeat the coin mechanism and obtain free merchandise. Many types of safety features have been developed over the years to prevent the theft of merchandise from bulk venders, including diameter measuring devices capable of measuring the diameter of a deposited coin to precise tolerances, which cooperate with the coin conveyor to prevent rotation of the mechanism unless the deposited coin is of the correct size.
A typical diameter measuring device comprises a dog pivotally mounted inside the cover plate, so that a coin of the correct diameter contacts the dog and raises the dog to a position at which the mechanism can be rotated about the dispensing cycle. This presents a difficult problem in a conventional coin mechanism. The amount of "play" in the mechanism, i.e. the extent to which the handle can be turned without the correct coin being deposited, must be minimal in order to avoid exposing merchandise to the entrance of the dispensing chute without the proper coin being deposited (commonly known as "milking" the vender). To avoid this the coin must be measured for proper size as close as possible to the beginning of the turning cycle. Thus, if the coin is not the correct size or if no coin has been deposited the handle cannot be turned far enough to expose merchandise to the dispensing chute.
However, this requires that the diameter measuring dog be positioned close to the coin slot. In such a coin mechanism the coin slot provides an access point into the interior of the mechanism. As the coin conveyor is rotated slightly past the rest position the coin recess provides an opening from the coin slot to the diameter measuring dog, such that a wire inserted through the coin slot can be manoeuvred to raise the measuring dog to the correct position, allowing the mechanism to be rotated without a coin being deposited.